The present invention relates to surface-modified pigments based on platelet-shaped substrates having improved settling and reagitation characteristics, and to their preparation processes and use.
Coating compositions, such as lacquers, paints, printing inks, etc., comprising pigments based on platelet-shaped substrates are subject to handling problems insofar as the pigments, owing to their size and density, settle readily and may then become compacted to form a highly solid sediment cake. This cake can generally be reagitated only with very great difficulty. This situation affects in particular the storage of lacquers, paints and printing inks and their processing. Furthermore, it is common when pigmenting plastics, basecoat systems, etc. to observe agglomeration of the pigments. Homogeneous distribution of the pigment in their respective matrix is difficult if not impossible to achieve.
Thus numerous methods have been developed, inter alia, in order to solve the problem of the incorporation and handling of platelet-shaped pigments in coating compositions.
DE 3627329 and the EP's 0306056 and 0268918 disclose that modified platelet-shaped substrates with a polymer coating and, respectively, following treatment with coupling reagents such as organotitanates, or organosilanes in covering compositions exhibit improved settling and reagitation characteristics.
Reagitation can also be facilitated by treating the coating compositions with additives, which bring about targeted flocculation (house of cards effect), pseudoplastic and/or thixotropic behavior, steric repulsion and/or electrostatic repulsion of the pigments.
Additives with thixotropic characteristics are described in EP 0198519 and DE-A 1805693. DE 3922128 discloses that mixing a suspension of a platelet-shaped substrate with spherical particles such as SiO.sub.2, TiO.sub.2 and ZrO.sub.2, for example, produces deagglomerated and readily dispersible pigments.
In EP 0523357, pearl luster pigments are treated with diurea derivatives, while EP 0515928 discloses the surface modification of pigments with polyacrylates. EP 0650144 discloses the coating of pearl luster pigments with SiO.sub.2 /Al.sub.2 O.sub.3 and SiO.sub.2 /ZrO.sub.2.
All of these additives may, however, have an adverse effect on the quality of the coating. In particular, the brilliance and color properties, in the case of pearl luster pigments, and the uniformity of the coating may be impaired. These impairments are generally greater the higher the concentration of additives employed.